A 20 MB scanned contract bounces off the mail server, or a government form rejects a passport scan over its limit. The fix depends on one thing: whether the PDF is real text or pictures of pages. Real text is already small and barely compresses; scans and photo-heavy exports are mostly images, and images are where the weight is. So the job is to make the images smaller without making the page unreadable.
One correction first. An earlier version of this page said the PDF tools on this site had a compress option. They do not; they convert images to PDF, PDF to images, merge, split and compress images. The routes below are the ones that exist, including how to use those tools for a scan.
Why PDFs get so big
A PDF that is just text is usually tiny. A 50-page text document might be well under 1 MB. So when a PDF is huge, the text is almost never the cause. The size comes from images.
Scanned documents are the worst offenders. When you scan a page, your scanner or phone captures it as a photograph, not as text. A single full-page color scan at high resolution can be several megabytes on its own. Stack ten of those together and you have a 20 MB file made entirely of pictures of paper.
The same thing happens with PDFs exported from design tools, presentations full of photos, or product brochures. Every embedded image carries its full resolution and its own encoding. Phone cameras and modern scanners capture far more detail than a screen or a printer actually needs, so most of that data is wasted weight. Compression works by trimming exactly this excess.
Route 1: re-export from the source
If you still have the document that produced the PDF, make the PDF again with smaller images. This keeps the text as text.
- Word: File → Save As, choose PDF, and set Optimize for to Minimum size (publishing online).
- Google Docs: images in a Doc are stored at reduced size already; File → Download → PDF Document usually gives a modest file.
- LibreOffice: File → Export As → Export as PDF, then on the General tab set JPEG compression quality lower and tick Reduce image resolution (150 DPI is plenty for screen reading).
- Design tools: export presets named for screen or web lower the image resolution and quality in one step.
Route 2: let the operating system do it
On a Mac, open the PDF in Preview, choose File → Export, open the Quartz Filter menu and pick Reduce File Size, then save under a new name. It downsamples the images and keeps the text, links and form fields intact. Check the result at full size: the built-in filter is aggressive on scans, and small print can go soft.
Windows has no built-in equivalent. Re-export from the source where you can; for a scan, use route 3.
Route 3: rebuild a scan from compressed images, in the browser
This route is for PDFs that are pictures of pages anyway, because it turns every page into an image and back. Three tools on this site do it without uploading the file:
- Open PDF to Images, add the PDF, choose JPG and the High (screen) resolution, and download the pages.
- Open the image compressor, add the page images, set a quality, or switch on Target size and give it a per-page budget, and download the compressed set.
- Open Images to PDF, add the compressed pages in order, choose a page size, and download the new PDF.
What you lose on this route is everything that was not a picture: selectable text, links, bookmarks, form fields and a digital signature. For a scanned receipt or a signed letter that was only ever an image, nothing is lost. For a report with real text, use routes 1 or 2.
Settings that work as a starting point, from the Arabic edition of this guide:
| Use | Start here |
|---|---|
| Reading on a phone | Pages 1280 px wide, quality 65 to 75% |
| Screen reading with small print | 1920 px wide, quality 75 to 85% |
| Print or archive | Keep the original, or re-export from the source |
| ID or contract | A local program; do not rebuild from images if the signature or form matters |
One trap: a PDF that is already small and made of real text can come out larger after this route, because high-resolution images of text pages weigh more than the text did. If the file is under a megabyte, it probably does not need compressing at all.
Route 4: fix it at the source, on the phone
Most oversized PDFs start as photos of paper. The DocFlow Scanner app I build scans with edge detection and cleanup and saves at a size meant for sharing, and its PDF tools include a compress option for files that are already on the phone. Scanning in document mode, grey or black-and-white where colour is not needed, is what makes a ten-page form come out small enough to email in the first place.
Habits that keep PDFs small
A few habits make a bigger difference than any single compression pass:
- Compress images before you build the PDF. If you are assembling a PDF from photos or screenshots, shrink those images first with our image compressor. A PDF built from already-light images starts small, so you may not need to compress the PDF at all.
- Scan at a sensible resolution. For a document meant to be read, 150 to 200 DPI is usually plenty. Many scanner apps default to 300 DPI or higher, which doubles or triples the file size for detail you will never see on a screen. Drop the resolution before you scan and the PDF is smaller from the start.
- Scan in grayscale or black-and-white when color is not needed. A grayscale scan of a printed page is far lighter than a color one, with no real loss for plain text.
- Remove pages you do not need. Blank pages, duplicate cover sheets, and extra appendices all add weight. Deleting them is the simplest size cut there is, and it costs nothing in quality.
How much smaller it gets
Image-heavy and scanned PDFs shrink a lot, because the file is mostly oversized images and every route above works on exactly that. Text-only PDFs barely shrink; if a compressed text PDF comes out almost the same size, that is expected, not a failure.
The trade is image quality against file size. Push any route hard and text in scans goes soft and small photos go blocky. Pick the lightest setting that still reads clearly at the size the recipient will view it, and keep the original until you have checked a few pages.